Grinding is a widely used precision machining process, accounting for over 20% of all machining processes in the manufacturing industry. Referring to FIG. 1, one type of grinding process employs a rapidly spinning grinding wheel 10 bonded with abrasive materials 12 (e.g., diamond abrasive particles in a resin, vitreous, or metallic bond). The wheel 10 grinds workpiece 14 moving slowly underneath the wheel 10.
Ceramic materials such as silicon nitride, silicon carbide, aluminum oxide, and zirconia are hard, low density materials with high wear resistance and the ability to withstand high temperatures. Grinding is often used to machine ceramic workpieces and workpieces made of other materials into their final shape. Costs associated with grinding include the cost of preparing a wheel (e.g., wheel truing and dressing).
Truing typically rounds a wheel by machining excess abrasive material off its periphery as the wheel rotates. Initially, a truing tool engages the rotating out-of-round wheel intermittently, removing material from protruding areas and progressively engaging more of the periphery as the wheel is rounded.
Dressing conditions the wheel surface topography to achieve a desirable grinding behavior. Typically, a bonded abrasive dressing stick is passed over the wheel periphery to expose the abrasive grains by eroding away binder and possibly removing and/or fracturing diamond grains. Re-dressing is periodically needed during grinding to recondition or resharpen a worn wheel surface. Severe and/or frequent dressing can result in excessive wheel consumption, whereas too gentle or insufficient dressing can result in a dull wheel. Dressing frequently can be time consuming and reduce the life of expensive abrasive materials. On the other hand, grinding with a dull wheel causes increased grinding forces which can lead to chatter vibration and damage to the workpiece.
For precision grinding operations, the wheel depth of cut may be comparable to or smaller than the wheel out-of-roundness. Therefore, wheel engagement with the workpiece can vary considerably during a single rotation. The wheel may even completely lose contact with the workpiece during part of each rotation. This unsteady behavior can have a deleterious effect on the wheel surface and the quality of the ground workpiece.
Material removal during grinding occurs when abrasive grains interact with the workpiece. This interaction generally involves both ductile flow and brittle fracture. As an abrasive grain engages the workpiece, initial cutting by ductile flow is followed by localized fracture if the grain depth of cut and the resulting force on the grain becomes sufficiently large. By analogy with indentation fracture mechanics, two principal types of cracks have been identified: lateral cracks which cause material removal and radial cracks which cause strength degradation. The implication of this observation is that strength degradation may be minimized by promoting ductile flow instead of fracture at the ground surface. For finish grinding operations, this would usually require extremely slow removal rates in order to achieve a small enough grain depth of cut and small enough force per grain. However, as a wheel is used and the abrasive material becomes duller, force levels increase, making it necessary to periodically re-dress the wheel. Periodic truing may also be necessary to restore the macroscopic shape of the wheel.
Typically, operators monitor the grinding and preparation processes to determine when the wheel is rounded and when the wheel needs to be dressed. Because of the practical difficulty in assessing the condition of a rapidly rotating wheel, operators typically manage wheel usage based on observation and experience. For example, an operator may periodically stop a grinding process to examine wheel characteristics (e.g., roundness and dullness) at intervals determined by the type of workpiece being ground.